1. Field of the Invention
The present invention relates to a drive apparatus.
2. Description of the Related Art
A stepping motor easily provides a high resolution without a position detector and thus is generally position-controlled in accordance with an open-loop control. However, when it is driven at a high speed, it may cause step out. Therefore, Japanese Patent Laid-Open No. (“JP”) 10-150798 proposes a motor control apparatus that provides a detector configured to detect a rotational position of a rotor and switches between an open-loop control that can provide precise positioning and a feedback control that can provide high-velocity driving.
The electrification is switched through an excitation to the coil, and the excitation current rises with a delay caused by the influence of the coil's inductance. Thus, as the rotor rotates faster, a phase switches and the torque lowers before the excitation current reaches a predetermined value. Accordingly, JP 10-150798 proposes to change, as the rotor's rotational velocity increases, a lead angle in accordance with the rotational velocity of the rotor, and to put forward an angle used to switch the excitation to the coil for early electrification to the coil. JP 09-331666 discloses one illustrative stepping motor.
Nevertheless, the motor control apparatus disclosed in JP 10-150798 can cause step out in switching the feedback control to the open-loop control. This is because a phase difference (or lead angle) between the electrification switching and the rotor position in the feedback control does not accord with that in the open-loop control.